1. Field of the Invention
This invention relates to a semiconductor laser module for use as a light source and a method for producing the same.
2. Description of the Related Art
As the number of channels of a cable television service increases, increasing attention is given to optical CATV (Community Antenna Television cable TV). In some communications, such as an optical CATV, a subcarrier multiplex (SCM) optical transmission system is adopted. The subcarrier multiplex optical transmission system is a communication system wherein a plurality of modulation signals frequency multiplexed in a radio frequency band are directly light intensity modulated and transmitted. The subcarrier multiplex optical transmission system is used, for example, for transmission of a multi-channel video signal of a CATV or a radio carrier of mobile communication.
Such a subcarrier multiplex optical communication system is required to have low distortion and low noise. It is demanded to use a semiconductor laser module having low distortion/noise characteristics different from those of a semiconductor laser which is used in an ordinary communication system for digital signals. The distortion/noise characteristics of a semiconductor laser module normally vary depending upon the bias current to be supplied to the semiconductor laser.
FIG. 1 illustrates a relationship of the bias current to the distortion characteristic and the fiber light output of a semiconductor laser. Referring to FIG. 1, a curve 11 indicates a relationship between the bias current and the distortion of the semiconductor laser, and another curve 12 indicates a relationship between the bias current and the fiber light output of the semiconductor laser. The distortion characteristic indicated in FIG. 1 is represented by an intermodulation second order distortion (IMD2) by a 2-tone method. The 2-tone method is a method wherein, in order to measure non-linear components, two different frequencies (f1, f2) are inputted simultaneously to a semiconductor laser and an intermodulation distortion produced by the semiconductor laser is measured.
As second and third order intermodulation distortions, distortions are produced in the following frequencies:
Second order intermodulation distortion: f1.+-.f2
Third order intermodulation distortion: 2.times.f1.+-.f2,
2.times.f2.+-.f1
As can be seen from the curve 11, if the bias current of the semiconductor laser is increased, then the distortion characteristic is improved as the bias current increases until it reaches to a certain current value I1. Then, as the bias current increases over this current value I1, the distortion increases and the distortion characteristic is deteriorated. On the other hand, as can be seen from the curve 12, as the bias current of the semiconductor laser increases, the fiber light output increases substantially linearly where the bias current is higher than threshold level current value I2. In this manner, there is a current condition or a fiber light output condition in which the semiconductor laser module shows its best distortion characteristic.
FIG. 2 illustrates a relationship between the fiber light output and the bias current and another relationship between the noise and the bias current of the semiconductor laser. Referring to FIG. 2, a curve 13 indicates a relationship between the bias current and the noise of the semiconductor laser and a curve 14 indicates a relationship between the bias current and the fiber light output of the semiconductor laser. The noise indicated in FIG. 2 is represented by relative intensity noise (RIN). As can be seen from the curve 13, as the bias current value of the semiconductor laser increases, the RIN decreases and the RIN characteristic is improved. The fiber light output of the curve 14 is same as curve 12 of FIG. 1. Accordingly, it can be seen that, as the fiber light output increases, the RIN characteristic is improved.
Generally, when a semiconductor laser module is used in a subcarrier multiplex optical transmission system, it is demanded to set the bias current to an optimum condition in order to achieve a desired distortion characteristic. However, since specifications of an ordinary transmission system include a limitation of a transmission loss between a transmitter and a receiver, the light output of a semiconductor laser module is specified so as to satisfy the specifications.
Accordingly, a semiconductor laser module cannot always be operated in an optimum current condition with regard to the distortion characteristic. Therefore, before a semiconductor laser module is adopted for a product, it is necessary to test the semiconductor laser module to detect whether or not they satisfy the required specification for selection.
FIG. 3 shows a construction of a conventional semiconductor laser module for a subcarrier multiplex optical transmission system. This semiconductor laser module 21 includes a semiconductor laser element 22, a lens 23 for converging emitting light of the semiconductor laser element 22, an optical fiber 24, and an optical isolator 25 interposed between the optical fiber 24 and the lens 23.
Light emitted from the semiconductor laser element 22 is converged by the lens 23 and optically coupled to the optical fiber 24. As the semiconductor laser element 22, a laser diode (DFB-LD) of the distribution feedback type which has high output, low distortion and low noise characteristics and oscillates in a single longitudinal mode is used popularly. Where a distribution feedback type laser diode is used as the semiconductor laser element 22, it is liable to be influenced by reflected returning light from, for example, an end face of optical fiber 24 or a junction of an optical connector (not shown) in a transmission line in the inside of the semiconductor laser module 21. As a result, the light output characteristic or the distortion/noise characteristics are liable to become unstable. Therefore, in the semiconductor laser module which employs a distribution feedback type laser diode, an optical isolator 25 is interposed in the optical system as seen in FIG. 3 to prevent the influence of reflected returning light upon the characteristics.
Conventionally, for the semiconductor laser module 21 of the construction shown in FIG. 3, a characteristic test to detect whether or not various characteristics required by a system are satisfied is performed and products which are acceptable are selected. Principal items of such a characteristic test are a light output, distortion and noise. Of the characteristics, particularly the distortion characteristic may possibly be much different between respective semiconductor laser elements 22 and is also much varied by the state of the optical coupling system in the module. Accordingly, when assembly of semiconductor laser module 21 is completed, a test must be performed to detect whether or not the distortion characteristic finally satisfies a predetermined specification.
Further, depending upon the method of use in the subcarrier multiplex optical transmission system, it is sometimes required for semiconductor laser module 21 to satisfy predetermined distortion/noise characteristics under various light output conditions. This is because the light output of the semiconductor laser module 21 is settled by the transmission loss based on the transmission distance between the transmitter and the receiver.
As a first method for solving such a problem as just described, a method is available wherein the optical coupling between the semiconductor laser element 22 and optical fiber 24 in the semiconductor laser module 21 is optimized to form them into a module. For example, a bias current condition for obtaining an optimum distortion value of the semiconductor laser element 22 is obtained in advance, and the optical coupling between the light output of the semiconductor laser element 22 itself and the optical fiber 24 under the bias current condition to form them into a module. In order to optimize the optical coupling, it is a common method to displace the relative positions of the each parts from their optimum positions to add an excessive loss.
FIG. 4 shows a semiconductor laser module which employs a second method for the solution of the problem. The semiconductor laser module 30 shown in FIG. 4 includes the optical fiber 24 of the semiconductor laser module having the same construction as that shown in FIG. 3 and an optical fixed attenuator 32 connected to the optical fiber 24 by a connecting element 31. In the semiconductor laser module 30 having this construction, an excessive loss is added by the optical fixed attenuator 32. The optical fixed attenuator 32 having a required attenuation amount is selectively secured for each semiconductor laser module 30. Further, since reflection of light occurs at connecting element 31, a connection method which achieves sufficiently low reflection must be selected in order that the characteristic of semiconductor laser module 30 may not be deteriorated by the reflected light.
FIG. 5 shows a further prior art (Japanese Patent Laid-Open No. 350981/1992 for achieving an operation with an optimum distortion characteristic. In an optical receiver 41 of a CATV optical transmission system shown in FIG. 5, an output of photodiode 42 is inputted to a signal amplifier 44 through a capacitor 43, and an output terminal of a signal amplifier 44 is connected to a band-pass filter 46. Thus, from the output signal of the signal amplifier 44, a desirable frequency is selected by the band-pass filter 46. A level detection circuit 47 is connected to the band-pass filter 46 and a discrimination circuit 48, and detects a distortion level of the frequency selected by the band-pass filter 46 and outputs a result of the detection to a discrimination circuit 48. A control signal generation circuit 49 is connected to the discrimination circuit 48 and a transmission apparatus 51 and transmits a control signal from the optical receiver 41 to an optical transmitter 52 through a signal line 53. A transmission apparatus 54 in the optical transmitter 52 is connected to the signal line 53 and a laser diode driving circuit 55. A transmission apparatus 54 controls the laser diode driving circuit 55 in accordance with the control signal from the control signal generation circuit 49 connected to the optical transmitter 52 by some transmission means, so as to minimize the distortion level of the optical receiver 41.
In the CATV optical transmission system shown in FIG. 5, the output signal from the signal amplifier 44 in the optical receiver 41 is branched, and a desirable frequency is selected by the band-pass filter 46. The level detection circuit 47 detects the level of the distortion of the selected frequency and the discrimination circuit 48 performs a discrimination based on a result of the detection. The control signal generation circuit 49 is connected to the discrimination circuit 48 and the transmission apparatus 51 and a transmits control signal from the optical receiver 41 to the optical transmitter 52 through the signal line 53.
Then, the control signal from the control signal generation circuit 49 is transmitted to the laser diode driving circuit 55 through the transmission apparatus 51 of the Optical transmitter 52. The control signal controls the laser diode driving circuit 55 so that a driving current for a laser diode 57 may be automatically set to a value with which the distortion is minimized in an arbitrary CATV optical transmission system. In this manner, the laser diode driving current can be automatically set to an optimum value. It is to be noted that, as shown in FIG. 5, the output side of an input terminal setting circuit 61 is connected to the laser diode 57 through a capacitor 62 and a resistor 63, and similarly, the driving side of the laser diode driving circuit 55 is connected to the laser diode 57 through a coil 64.
As described above, in the conventional semiconductor modules, a test for discriminating whether or not characteristics are acceptable is performed for finished products. Consequently, there is a problem in that, if the ratio of rejected products is high, it makes the production cost higher. Further, there is another problem in that, if it is tried to apply products which do not satisfy the specification as products of a different less severe specification, then a selecting operation is required, which increases the complication in production management.
Further, where a semiconductor module is required to satisfy predetermined distortion/noise characteristics under various light output conditions, it is necessary to add an optical coupling loss to the module in accordance with the required light output condition and the distortion characteristics, during the production process of the module. Therefore, a plurality of products produced in different production conditions must be prepared, and this complicates production management since one kind of product is produced in at least one lot for each production condition. In particular, since the bias condition which makes the distortion characteristic optimum is different between different semiconductor laser elements, it cannot be avoided to discriminate whether or not the distortion characteristic is satisfied in a desired light output condition by a characteristic test after the semiconductor laser module is completed, resulting in complication.
Further, with the arrangement illustrated in FIG. 4, several different optical fixed attenuators 32 must be prepared in advance taking required attenuation amounts into consideration. Therefore, there is a problem in that this results in complication in production management. This is because, since the bias condition which provides an optimum distortion characteristic is different between different semiconductor laser elements similarly as in the case of the semiconductor laser module described above, whether or not a distortion characteristic is satisfied in a desired light output condition does not become clear until after a discrimination whether or not a semiconductor laser module is acceptable is performed after completion of the semiconductor module.
It is to be noted that, in the prior art disclosed in Japanese Patent Laid-Open No. 350981/1992 and shown in FIG. 5, the bias current value of the semiconductor laser module is varied in order to improve the distortion characteristic. Consequently, as the bias current value varies, the light output level varies simultaneously, and as a result, an operation in a fixed light output condition cannot be performed any more. Therefore, the problems described above cannot be solved essentially.